With any multi-gpu setup you're only as strong as your drivers. Some games are going to be great, others aren't going to scale well (50% more fps), some aren't going to scale (0% gained fps) and some will actually pull you back due to the excess processor overhead (-%).
An Eyefinity setup at 5760x1080 is probably going to hit the wall at VRAM usage before the GPUs run out of grunt. 7770 only have 1GB VRAM per card.
If you already have a 7770, you'd probably be better off stepping up to a better card.
According to this chart...http://www.techpowerup.com/reviews/HIS/HD_7750_7770_CrossFire/22.html
A high end 6 series, or a 7870, 7950 or 7970 is going to suit you far better.
But if you don't want to take the hassle of selling your 7770 and upgrading, another one PROBABLY isn't going to hurt.